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Alternator BOSCH in VW 70A, 90A, 120A, 140A how do they differ?

tata1 18885 12
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  • #1 16955559
    tata1
    Level 20  
    Hello
    The question applies to Bosch alternators mounted in the VW group - how do the different models differ?
    In appearance they are identical, when they are dismantled they are also identical. The mountings are the same, they can be mounted interchangeably - and the difference is in the max charging current:

    028 903 028C - 70A
    028 903 028D - 90A
    028 903 028E - 120A
    06F 903 023J - 140A

    I thought the max charging current depends on the voltage regulator. But it doesn't matter, because in 90A and 120A I have F00M145225, and in 70A I have F00M144128 and it turns out that this is a replacement model for that one. Similarly in 140A I have F00M145296 and he is also a replacement for those.

    What in the construction of these alternators affects the max charging current?
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    #2 16955603
    Anonymous
    Anonymous  
  • #3 16958702
    grzegorzbuczylo
    Level 20  
    Sometimes the number of diodes in the rectifier system and the number of winding leads. For posterity, whoever asks, I have a 120A alternator in the Passat b4 1.9tdi and I installed the VARTA AGM 92AH battery and I have been driving for a year and there is no problem with recharging.
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  • #4 16958721
    Anonymous
    Anonymous  
  • #5 16958780
    robokop
    VIP Meritorious for electroda.pl  
    The diameter and width of the winding yoke, and hence the thickness of the wire used, the magnet (rotor) then will also have a different diameter and length. Due to the different magnetizing current characteristics, you will have other regulators - probably in terms of switching frequency and allowable current.
    grzegorzbuczylo wrote:
    there is no problem with topping up
    And why should they be? In tractors you have 55-75A alternators and 170Ah batteries - somehow there are no problems. In my private dump truck I have a 45Ah battery and a 120A alternator, and I also play without failure?
  • #6 16958792
    grzegorzbuczylo
    Level 20  
    Well, I wrote because there is always a question whether he tops up.
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    #7 16958818
    Strumien swiadomosci swia
    Level 43  
    tata1 wrote:
    In appearance they are identical, when they are dismantled they are also identical


    Well, that you drank sharply, buddy, everyone has a different rotor, the higher the current, the larger the rotor, and hence the larger the stator.
    120 140 has the same rotor but different diodes.



    Bravo zero perceptiveness.
  • #8 16958949
    Stanisław Chwalisz
    Level 34  
    Alternators increase the current because they differ in structure. They are made as 3-phase, 6-phase, 9-phase and 12-phase and hence greater current.
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  • #9 16961167
    tata1
    Level 20  
    Strumien swiadomosci swia wrote:
    tata1 wrote:
    In appearance they are identical, when they are dismantled they are also identical


    Well, that you drank sharply, buddy, everyone has a different rotor, the higher the current, the larger the rotor, and hence the larger the stator.
    120 140 has the same rotor but different diodes.



    Bravo zero perceptiveness.


    Not zero perceptiveness, only when I replaced bearings in one, at a time I had only dismantled one - hence they seemed to me the same. That is why I set up this topic, because I was curious how the hell they differ, since they are the same in appearance. They have an identical arrangement of diodes, the rear plastic cover physically fits on each of these models interchangeably.
    I have not measured the caliper wire, as well as the diameter of the rotor or the entire alternator, but the differences must be small, because you can not see them by eye.
    Now I just take the bearings in this 140A in Sharan, and the car must drive, I threw a 90A there temporarily and that's where the topic was because I thought 140 would be visually bigger, and this chick ... similar.
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    #10 16961184
    andrzej20001
    Level 43  
    More coils and their other diameter, hence greater power.
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    #11 16961234
    robokop
    VIP Meritorious for electroda.pl  
    Stanisław Chwalisz wrote:
    Alternators increase the current because they differ in structure. They are made as 3-phase, 6-phase, 9-phase and 12-phase and hence greater current.
    Even 20 or 50 phase will not have a larger rated current without increasing the core dimensions - the excitation flux is still the bottleneck. And then heat dissipation from the whole.
  • #12 16961240
    andrzej20001
    Level 43  
    Because of this are "hydrocele" :)
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  • #13 16961254
    tata1
    Level 20  
    Quote:
    Alternators increase the current because they differ in structure. They are made as 3-phase, 6-phase, 9-phase and 12-phase and hence greater current.
    Even 20 or 50 phase will not have a larger rated current without increasing the core dimensions - the excitation flux is still the bottleneck. And then heat dissipation from the whole.


    Yes, they differ in size - the diode arrangement is identical

    In fact, when you put them next to each other, you can see how 140A is more "inflated" in relation to 90A. This is especially visible when you put them on the pulley and look from the side of the plastic diode cover. In 140A the body protrudes from the cover more than in 90A.
    I was deceived by the fact that they have the same diode covers and physically fit interchangeably, and I was confused by the fact that they are roughly identical.
    However, the forum is to figure out the details - thanks for your help.
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Topic summary

✨ The discussion centers on the differences between Bosch alternators used in VW vehicles, specifically models with varying maximum charging currents: 70A (028 903 028C), 90A (028 903 028D), 120A (028 903 028E), and 140A (06F 903 023J). While the alternators appear identical externally and can be interchanged, their internal construction varies. Key factors affecting the maximum charging current include the thickness and number of winding wires, the number of diodes in the rectifier system, and the dimensions of the rotor and stator. Higher current models have larger rotors and stators, which contribute to their increased power output. The discussion also notes that while the diode arrangements may be similar, the physical size differences become apparent when the alternators are compared side by side.
Generated by the language model.

FAQ

TL;DR: VW Bosch alternators that look identical still vary by output (70A–140A) due to internal rotor/stator and rectifier differences. As the OP notes, “the difference is in the max charging current.” [Elektroda, tata1, post #16955559]

Why it matters: This FAQ helps VW owners and techs pick, swap, or diagnose Bosch alternators without guesswork.

Quick Facts

What actually determines the max charging current on these Bosch alternators?

Core dimensions and copper do. Larger rotor/stator iron and wider yoke allow more flux and copper, raising output. Rectifier and regulator choices then align to the higher allowable current and heat. “The higher the current, the larger the rotor, and hence the larger the stator.” [Elektroda, Strumien swiadomosci swia, post #16958818]

Are the 70A, 90A, 120A, and 140A units physically different or just the regulators?

They are physically different. Higher-amp versions use larger rotor/stator packs, and 140A strengthens the diode pack compared with 120A. Regulators are matched to the magnetizing current and switching demands, not the only difference. [Elektroda, Strumien swiadomosci swia, post #16958818]

Does increasing the number of phases automatically increase alternator current?

No. More phases alone do not raise the rated current. The excitation flux and heat removal limit current first; iron and cooling capacity dominate. Extra phases help ripple and packaging, not the ceiling. [Elektroda, robokop, post #16961234]

Can I swap a 90A alternator into a car that came with 140A?

Yes, as a temporary or light-duty solution. Mounting points match, and users report direct swaps to keep the car running. Expect less headroom for accessories under heavy loads. [Elektroda, tata1, post #16961167]

Will a 120A alternator overcharge a big AGM battery?

No, not if the regulator and wiring are healthy. One user runs a 120A alternator with a VARTA AGM 92Ah for a year without recharge issues. “There is no problem with recharging.” [Elektroda, grzegorzbuczylo, post #16958702]

Could a higher-output alternator ‘cook’ an even larger battery?

Within reason, yes—the system can support larger batteries when regulated correctly. A forum reply notes the 120A unit can handle an even larger battery without harm in normal use. [Elektroda, 767667, post #16958721]

Is there a quick visual check to tell 140A from 90A?

Yes. Viewed from the pulley side toward the rear cover, the 140A body protrudes farther, giving a slightly “inflated” look versus 90A. [Elektroda, tata1, post #16961254]

Do regulators differ across these alternators?

Yes. Because magnetizing current and response differ, regulators vary in switching behavior and permissible current. Higher outputs require suitable regulator characteristics and thermal margins. [Elektroda, robokop, post #16958780]

Can small alternators maintain large-capacity batteries?

Yes, given realistic loads and drive time. One expert cites tractors with 55–75A alternators feeding 170Ah batteries without issues. That highlights usage patterns over raw capacity. [Elektroda, robokop, post #16958780]

What do ‘rotor’ and ‘stator’ mean here?

In this thread, the rotor is the excited magnet assembly; the stator is the winding yoke that sets output with wire size and width. Both scale with target current. [Elektroda, robokop, post #16958780]

How do winding choices impact output?

Thicker wire and more turns per phase reduce resistance and raise current capability, within the iron’s flux limits. Or as one reply states, “Winding wire thickness and amount.” [Elektroda, 762302, post #16955603]

Do rectifier diodes and winding leads vary between models?

Yes. Users report differences in diode count and the number of winding leads, aligning the rectifier to the alternator’s current rating. [Elektroda, grzegorzbuczylo, post #16958702]

Are Bosch regulator part numbers interchangeable across outputs?

The thread notes F00M145225 and F00M144128 used on 70A–120A, and F00M145296 on 140A, described as replacements by the OP. Verify on the unit label before purchase. [Elektroda, tata1, post #16955559]

What’s the main failure or edge case when pushing output?

Thermal limits. Even with clever phasing, excitation flux and heat dissipation cap safe current. Overheating can stress diodes and insulation under sustained high load. [Elektroda, robokop, post #16961234]

How can I compare two VW Bosch alternators quickly (90A vs 140A)?

  1. Check physical massing: higher output looks slightly larger around the stator window.
  2. Inspect rectifier: 140A uses a stronger diode pack than 120A.
  3. Note that rotor size tracks current rating on these families. [Elektroda, Strumien swiadomosci swia, post #16958818]
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